It has recently been hypothesized that the spatial representation of frequency along the cochlea changes during development. This hypothesis predicts that the tonotopic organization within the central auditory pathway should shift during ontogeny. To test this, the tonotopic organization of the avian brain stem auditory nuclei n. mangocellularis and n. laminaris has been mapped during embryonic and postnatal development. The proposed experiments are a continuation and extension of these studies. In the first two experiments electrophysiological mapping procedures will be used to (1) complete the mapping of tonotopic organization in the 17 day embryo, and (2) map the tonotopic organization during the earliest stages of functional development. The completion of these studies will provide a complete quantitative description of the development of tonotopic organization in n. magnocellularis and n. laminaris. In the third experiment the horseradish peroxidase technique for tracing neural connections will be used to determine if the projections between the basilar papilla (cochlea) and n. magnocellularis change during development. This will determine whether the changes in tonotopic organization which have been observed can be explained by a remapping of connections between the basilar papilla and n. magnocellularis. The functional consequence of a change in the spatial encoding of frequency along the cochlea is that during development the same acoustic stimulus will maximally activate different subpopulations of neurons within an auditory nucleus. This may have important implications for understanding how the early acoustic environment influences auditory system development. A change in the spatial encoding of frequency may also have important clinical implications. If the relationship between place along the cochlea and frequency is not necessarily fixed, then this relationship might also change during aging or be altered by various conditions that result in hearing impairment and auditory system dysfunction. These possibilities need to be examined in future studies.